The present invention relates generally to gas turbine combustors and more specifically to a fuel and air mixing device in a gas turbine combustor.
In a gas turbine engine, the combustion section contains a reaction that occurs when fuel and compressed air are mixed together and react after being ignited by an ignition source. Compressed air is directed to one or more combustion chambers from the engine compressor. Fuel injection devices inject a fuel, either liquid or gas, into the compressed air stream and the mixture undergoes a chemical reaction once being exposed to a heat source, such as an igniter.
Some examples of prior art mixer devices are shown in FIGS. 1 and 2. FIG. 1 is a cross section of a combustion system disclosed in U.S. Pat. No. 5,515,680, and hereby incorporated by reference. The combustion system utilizes a ring member 31 to inject a fuel transverse to the flow direction of the premixing combustion air, at the outside of a 180 degree bend in the air flow path, in an effort to inject the fuel from a high velocity region towards a lower velocity region for improved mixing. While this technique may improve mixing locally, further improvements can be made such that additional time and distance is provided in the region upstream of the combustor to further enhance premixing. FIG. 2, on the other hand, is a cross section of a fuel injector and mixing device disclosed in U.S. Pat. No. 5,165,241 that injects a fuel from the centerbody of the injector, radially outward into the passing air stream, which has previously undergone counter rotating swirl from inner swirler 26 and outer swirler 28. While this type of mixer attempts to provide improved premixing, it too can be improved by providing a longer time and distance for the fuel and air premixing to be more complete prior to ignition.
In order to control emissions levels of oxides of nitrogen (NOx) and carbon monoxide (CO), it is critical that the fuel molecules burn as completely as possible such as to not leave any unburned hydrocarbons to pass into the atmosphere. In order for the fuel to completely burn a number of issues must be addressed, one of which is fuel and air mixedness prior to ignition. Fuel and air mixedness is controlled by factors such as swirl, fuel injection location, and mixing time prior to ignition. Therefore, for the lowest possible emissions, it is most desirable to provide a mixer for a gas turbine combustor that optimizes swirl, fuel injection location, and mixing time such that the combustion process will be as complete as possible.